1. Field of the Invention
The present invention relates to apparatus for positioning an article on a work surface at predetermined X, Y plot coordinates, or dynamically at a continuous succession of such coordinates, and more particularly to an article positioning apparatus having one or more positioners coupled to an article by a tape or tapes sufficiently longitudinally rigid that the article can be positioned by extending or retracting the tape.
2. Description of the Related Art
There are various applications in which it would be advantageous to be able to precisely locate an article at predetermined X, Y plot coordinates, and to do this dynamically so as to continuously trace a pattern upon a work surface.
In the sign making trade large signs could be made in this way. At present the letters of such signs are traced on sign material using large templates to establish the X, Y coordinates for the marking instrument or stylus. Sometimes a pantograph arrangement is used to trace the large letters from a smaller pattern, or an optical projection system is used to project the large letters from a smaller pattern.
Precise location of a stylus or the like is also important in making drawings. Heretofore, positioners in the form of X-Y plotters have been used for this purpose. Such a plotter typically employs a carriage movable along the X axis, and mounting a stylus movable along the Y axis. Another form of plotter utilizes a telescoping arm movable in and out and rotatable in an arc about its pivot to place the marking stylus at the desired X, Y plot coordinates. The size and weight of the components of such plotters introduces an inertia which adversely affects sensitive response to actuating forces. Moreover, the larger the image to be drawn, the larger must be components, such as the lead screws to move the carriage and stylus, or the telescoping arm.
A cable driven form of plotter designed to reduce the undesirable inertia of the foregoing plotter types is described in U.S. Pat. No. 4,412,383, issued to Landa and entitled "Cable Driven Plotter". A pair of cables coupled to a stylus are movable in and out from reels which are pivotally carried at one edge of a plotting board. The extent of cable movement is controlled to locate the stylus at the desired X, Y coordinates. A carriage freely movable along the opposite edge of the plotting board is connected by a third cable to the stylus. The latter cable is spring biased to place the other pair of cables under tension. This takes up slack and promotes greater precision of movement. Another form of cable driven plotter is disclosed in U.S. Pat. No. 4,583,292, entitled "Low Inertia X-Y Cable Drive". The plotter employs four flexible cables attached to a stylus. Each cable is changed in length by a separate digital servo drive motor which responds to signals from a control means comprising an external computer and microprocessor. The control signals correspond to desired plot coordinates X, Y stored in the control means. Sensors associated with the cable pulleys provide cable length feedback signals which are input to the computer. This plotter, according to Langberg, is fundamentally different from the Landasystem in that "the cables must be under at least minimum tension so that the marker housing is suspended above the plotting medium and there is no slack in cables when the motion is started again". Also, Langberg provides a system of force sensors to monitor the tension of all cables, and provide corresponding feedback signals to the servo control system.